In this activity, students become familiar with radio waves that are used to remotely sense the topography beneath the ice sheet. They experiment with travel time of waves and convert these to distance. The students, in groups, examine time data acquired along profiles of the Antarctic ice sheet and convert these data to depth, resulting in a profile of the topography beneath the ice sheet. The students "pool" their profiles to get a better view of the topography beneath the ice sheet. They compare their findings with radio-echo soundings of the ice sheet and with the map of sub-ice topography. Students contrast this method of data acquisition with that of coring (What's Under There?).
Maps of the Antarctic continent showing the land under the ice have been difficult in the past to make. Today, with the use of satellites, almost our whole earth and even our sky are being mapped using the scientific instruments involving electromagnetic or sound waves.
6th grade and higher, Earth Science, Physical Science, Physics
The student will:
Teacher Preparation for Activity
Place each of the three maps of Antarctica on a separate wall. The students will approach these maps in groups to examine them; the class also will discuss the maps.
For each group of 4 to 5 students:
Two class periods
Engagement and Exploration (Student Inquiry Activity)
As a class, discuss the findings of the students (refer to questions). How do the students think the maps were constructed? Do we have data for every single point on the map? Were cores used? Are there other ways to acquire information?
Researchers get information about the thickness of the ice sheet and depth to basement through cores - a data collection method similar to the way students collected topographic information in their mystery boxes. They also use other techniques, using sound waves and electromagnetic waves. Some sound waves we can hear; electromagnetic waves includes light in the spectrum we see.
The researchers using electromagnetic waves to image through the ice sheet use radio waves. The technique is called "radio-echo sounding" or RES. The more recent term applied to the tool is "ground penetrating radar" or GPR.
Scientists use devices to generate radio waves at the surface of an ice sheet. The radio waves travel through the ice sheet. When the wave "hits" a surface, such as a layer of more dense ice, or a pool of water, or the rock surface under the ice, part of it bounces back to the surface where the scientists record the return. The scientists can figure out where the object is under the ice using the amount of time the wave traveled to the object and back. They can change "time" into depth (or thickness) because they know how fast the waves travel. Actually, there are computers that make this conversion for the scientists; the data are displayed as a radar image. The students will look at GPR data from the ice sheet later in the activity.
The time has to be divided in half because the time the scientist records includes the wave's trip to the object and the wave's trip back to the surface.
Elaboration (Polar Applications)
Provide each group with a slinky, a measuring tape, a stopwatch, graph paper, pencils. Have each group work near a wall. The students will explore the d=vt relationship by creating waves with the slinky and timing how long it takes for the wave to travel to the wall and back to the student generating the wave.
Ask the students to measure 3 meters from the wall. One student can sit close to the wall and one student can sit at the 3 meter mark. Have the students quickly move the slinky to the right and then to the left so that a wave is created. What happens to the wave?
This is similar to how waves act in the ice sheet. The scientists have an instrument that generates the wave. The wave travels through the ice (across the floor), hits the rock beneath the ice sheet (the other student), and then bounces back to the top of the ice sheet (the first student).
Teacher Note: The side-to-side wave is similar to a transverse wave. Light, radar, ultraviolet, and TV waves are transverse waves. They are all members of the electromagnetic spectrum. By creating a "pull and push" motion with the slinky on the floor, students can simulate longitudinal waves (compressional). This models how sound waves travel. While GPR uses radar waves, other methods to image the ice sheet employ sound waves. The results are similar.
Ask the students to measure the time taken by the travel from the student generating the wave and back again. Have the groups measure this same distance several times. What parts of the experiment do we want to keep constant? The students should try to generate the waves in a consistent fashion. Why do we want to repeat the experiment several times? Make sure the students record their data.
Next, have the students repeat the experiment with the students 2.5 meters from each other. Record the data for three trials. Repeat for 2, 1.5, 1, 0.5 meters. What happens to the speed of the wave as the students get closer and closer? At some point, the students may not be able to measure the difference.
Have the student groups graph their results of distance and time. Remind them that they want the time for the wave to travel one way, just like scientists only want the time the wave travels to the rock under the ice sheet. They need to cut the time in half.
How do they want to show the multiple trials? Suggest that they average the three trials so that they get a single number to plot.
Ask the students to calculate how fast the wave was traveling using the equation:
How can they find the speed of the wave? They will need to move the parts of the equation so that:
What trends do they see? Does the speed vary for the trials? Probably not by much. For older students, this can be discussed in the context of the slope of the line that is created by plotting depth versus wave travel time.
When the scientists are measuring the thickness of the ice sheet, or the depth to the bottom, they know the speed of the wave in ice and the time that it takes for the wave to return.
Exchange (Students Draw Conclusions)
Have the students present their graphs to the class. Are all the graphs similar? Why or why not? What controls how fast a wave returns to the "surface"?
If the scientists were measuring radar waves on the ice sheet, and the wave returned very quickly from the rock surface beneath the ice sheet, how close would the rock surface be? Close? Far away? If the wave took much longer to bounce back, how close would the rock surface be? Close? Far away?
Give each student group a RES image. Show them on the map where the image was collected. What do they see? Can they identify the surface of the ice sheet? The bottom? The rock under the ice sheet? The layers inside the ice sheet? This image was collected by recording radar waves that penetrated the ice sheet and then bounced back to the surface.
Return to the maps. Ask the students how the data to make the map were collected. Some students may recall the coring activity. Much of our information about the thickness of the ice sheets and the sub-ice topography is gained by data collected by techniques illustrated in this activity - remotely sensing the ice sheet with radar and sound waves. Ask the students why coring is still very important. While more expensive, coring provides new information about the ice sheet and it provides "ground truth" - evidence that the data from remote sensing is interpreted correct.
Evaluation (Assessing Student Performance)
Sandra Shutey, Butte High School, Butte, Montana; Stephanie Shipp, Rice University, Houston, Texas; Kristen Bjork, Educational Development Center, Newton, Massachusetts
The International Geophysical Year (IGY) of 1958 marked the initiation of focused investigations into the structure of Antarctica. Our knowledge of the continent increased dramatically as a result of these studies. We continue to investigate the continent with ever-changing technology. During the early years, scientists used explosives to generate sound waves. Charges of dynamite were placed in the ice and exploded. The shock waves, recorded on a seismograph, penetrated the ice and the rock below. The waves bounced off the different layers and returned to the surface where they were recorded by the sensors (Image). Changes in the rate of travel of the waves passing from ice to rock allowed scientists to determine the ice-rock boundary and to measure the true depth of the ice. From these data, scientists created cross-sections that revealed the great thickness of the ice.
Today, scientists use techniques such as radio-echo sounding (RES) to collect data about ice sheet thickness inexpensively and quickly (also a little less dangerously!) (Image). RES involves either sledding or flying over the ice sheet while transmitting and receiving radio signals. The data used in this activity are RES profiles.
Radio-echo sounding (RES) techniques are relatively new to the field of glaciology. A radio pulse between the frequencies of 35 and 300 MHz is transmitted from an instrument at the ice surface. The pulse penetrates the ice and reflects from internal layers and the ice/substrate contact back to surface listening devices. The returned signals are recorded and processed digitally into high-resolution images of the internal structure and thicknesses of ice sheets.
The potential to use RES as a method to measure ice thickness came about from some accidental airplane landings on the Greenland Ice Sheet. During WWII military aircraft had radar devices to indicate their clearance above the ground surface. Ice is partly transparent to radio waves, and portions of the radio signal penetrated the ice sheet and bounced off the rock surface underneath. Thus the radar provided the clearance above the bed of the ice sheet and not the surface of the ice sheet! In consequence, airplanes made unintentional wheels-up landings on the snow on top of the ice sheet. No one was hurt, but scientists learned about this new method.
RES data allow scientists to look into the ice sheet and see its base in a continuous profile. But the RES profiles tell glaciologists much more than just the thickness of the ice sheet. RES data show layers within the sheet that can be traced across great distances; each layer is a time line. These help glaciologists understand how the ice is flowing. By comparing RES profiles at the same location from different times, researchers can trace the movement of a parcticular pattern or feature and can determine how fast the ice sheet is moving. RES data can even provide information about the temperatures of ice sheets! Radio waves travel at different speeds through ice of different temperatures; these differences can be interpreted to give glaciologists more information about the ice sheet and its activity. Based on the pattern of the returned signal, glacial geologists and glaciologists have identified different types of rock under the ice sheet, the locations of subglacial lakes, and the presence of large crevasses at the bases of ice shelves.
Based on RES and seismic surveys of the Antarctic ice sheet, a better understanding of the size, volume, and conditions within the ice sheet has emerged. RES data provide scientists with a method of data collection that is relatively rapid, and more continuous and less costly than other types of surveys. Remember, however, interpretations of the data must be field tested. Ice cores and other types of sampling are invaluable for verifying that the interpretations are correct!
Background modified from GLACIER supplementary curriculum. Materials are available through GLACIER.
Employing Remote Sensing in the Field:
Kim Giesting Journals; Oceanography
Hubble Space Telescope; Astronomy
Antarctic sub-ice topographic, thickness, and surface elevation maps are available from the Scott Polar Research Institute.
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